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MOSFET Question

mickey.zhang
Prodigy 40 points

Hi team,

I want to use a BJT to control a NMOS. I have a TINA similuation. 

Q1, Why AM1 current is a uA current? This uA current is the MOS driver current? How to calculate the mos driver current in this circuit? 5V/100 is the MOS driver current?

Q2, In this circuit, I use 2 BJT to control a NMOS. Is there any way to use only one BJT to control a NMOS?

MOS.TSC

Q3, When I use another 2 BJT to control the NMOS. If BJT input is 3.3V, NMOS is closed and the IDS of MOS is cloesed to 0.  Why NMOS cannot be open? How to improve this circuit?

I refer to https://www.ti.com.cn/cn/lit/ml/slua618a/slua618a.pdf?ts=1661079800607&ref_url=https%253A%252F%252Fwww.ti.com.cn%252Fsitesearch%252Fzh-cn%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Dzh-CN%2526searchTerm%253DSLUA618%2526nr%253D33

MOS - 2.TSC

  • 0
    TI__Mastermind 34275 points

    Hello Mickey,

    Thanks for your interest in TI FETs. To answer your questions:

    A1. The current thru AM1 = VF1/(R2 + R3) = 4.99V/(100Ω + 10kΩ) = 494μA. A small amount of leakage current will flow from gate-to-source usually in the 10's of nA up to μAs depending on the FET selected. The gate drive current is limited by the gate resistor, R2, and will be higher during switching transitions where the FET input capacitance is charged and discharged by the gate drive circuit.

    A2. You can drive the gate of the FET with a single NPN transistor by connecting the gate to the collector and using a pull up resistor to the supply voltage, in this case 5V. When the NPN is on, the gate of the FET is pulled LOW and it is off. When the NPN is off, the gate of the FET is pulled up to 5V thru the resistor.

    A3. I ran the DC analysis on the totem pole circuit. When V2 = 3.3V, the gate voltage is 2.65V which doesn't appear to be enough voltage to fully turn on the FET, IRF120, used in your simulation. If V2 = 5V, then the gate voltage is 4.32V which is enough to turn on this particular FET. When V2 = 0V, the gate voltage is 56nV and the FET is off.

    Depending on your application, there are many discrete options to drive the FET and there are also integrated gate drivers from TI available at the link below.

    https://www.ti.com/power-management/gate-drivers/overview.html

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    Prodigy 40 points

    Hi John,

    For your A1, The gate drive current  is AM1 = VF1/(R2 + R3) = 4.99V/(100Ω + 10kΩ) = 494μA?  this current cannot drive a mos? 

    How to increase the gate drive current? Remove R3 can increase the gate drive current? But once remove R3, mos  may be turned on by mistake。

  • 0 in reply to mickey.zhang
    TI__Mastermind 34275 points

    Hi Mickey,

    I calculated the steady current thru AM1 after the gate has been charged up to 5V. Please refer to page 3 of the app note in the link below for more information on calculating gate current. Reducing the value of R2 will increase the gate current and speed up turn-on and turn-off. It can be 0Ω if desired. You can also increase the value of R3 to reduce the current shunted away from the gate. This can be 100kΩ to 1MΩ. What is the application? Is the FET being used as a load switch or in a switch mode power supply? In load switching or hot swap applications, switching speed is not critical like a DC-DC converter. Let me know if I can be of further assistance.

    https://www.ti.com/lit/an/slpa009a/slpa009a.pdf

    Thanks,

    John

  • 0 in reply to John Wallace1
    Prodigy 40 points

    load is a Motor. Use the NOMS to driver motor. MCU to control the gate of mos directly, is this way reliable？

  • 0 in reply to mickey.zhang
    TI__Mastermind 34275 points

    Hi Mickey,

    Thanks again for your interest in TI FETs. Driving a FET directly with MCU output can work but may be difficult due to limited current drive capability and VCC used to power the MCU. An external drive circuit, integrated or discrete, may be necessary to achieve the desired performance. Without knowing your specific application requirements, it is difficult to make a recommendation.

    Best Regards,

    John